LMH6555EVAL National Semiconductor, LMH6555EVAL Datasheet - Page 13

• Manufacturer Part Number: LMH6555EVAL
• Manufacturer: National Semiconductor

Specifications of LMH6555EVAL

• Lead Free Status / Rohs Status: Not Compliant

DIFFERENTIAL INPUT
The following is the procedure for determining the device op-
erating conditions for differential input applications using the
Figure 5 schematic as an example.
Assuming transformer secondary, V
1. Calculate the swing across the input terminals
2. Calculate each input pin swing to be ½ the swing
3. Determine the DC average of the two inputs (V
4. Calculate V
(V
differential source (V
with differential input impedance R
For Figure 5
determined in step 1:
For Figure 5
using the following expression:
For Figure 5
For Figure 5
V
V
V
V
V
where R
to the LMH6555)
R
V
V
A
where R
R
V
IN_DIFF
IN_DIFF
IN_DIFF
IN
IN
I_CM
I_CM
OUT
OUT
S
V_DIFF
S
+ = V
+ = V
= 50Ω
= 50Ω
= (V
= (0.3 V
= 12.6 mA · R
= 15.75/ (50+64) = 138.2 mV
FIGURE 5. Differential Input Drive
) by considering the voltage division from the
= 2 · R
= V
= 300 mV
IN
IN
E
F
= 430Ω & R
IN
− = V
− = 131.5 mV
= 25Ω & R
→
OUT
→
IN
· / 2) · A
V
A
PP
· R
I_CM
knowing the Insertion Gain (A
V_DIFF
F
IN_DIFF
/ (2R
/2) · 4.83 V/V= 724.5 mV
IN_DIFF
PP
= 15.75 / (R
IN
V_DIFF
E
S
= 4.83 V/V
· 78 / (100 + 78) = 131.5 mV
G
) to the LMH6555 input terminals
IN_DIFF
/ 2
· R
+ R
= 39Ω (both internal
IN
/ (2R
PP
, of 300 mV
S
IN_DIFF
/ 2 = 65.7 mV
/ (R
S
= 78Ω
+ R
S
S
+ 64)
+ R
)
IN_DIFF
IN_DIFF
PP
G
+ R
:
)
PP
PP
E
)
20127720
V_DIFF
I_CM
PP
) by
):
13
The values determined with the procedure outlined here are
shown in Figure 6.
SOURCE IMPEDANCE(S) AND THEIR EFFECT ON GAIN
AND OFFSET
The source impedances R
or Figure 5, affect gain and output offset. The datasheet tables
and typical performance graphs are generated with equal val-
ued source impedances R
ified. Any mismatch between the values of these two
impedances would alter the gain and offset voltage.
OUTPUT OFFSET CONTROL AND ADJUSTMENT
There are applications which require that the LMH6555 dif-
ferential output voltage be set by the user. An example of such
an application is a unipolar signal which is converted to a dif-
ferential output by the LMH6555. In order to utilize the full
scale range of the ADC input, it is beneficial to shift the
LMH6555 outputs to the limits of the ADC analog input range
under minimal signal condition. That is, one LMH6555 output
is shifted close to the negative limit of the ADC analog input
and the other close to the positive limit of the ADC analog
input. Then, under maximum signal condition, with proper
gain, the full scale range of the ADC input can be traversed
and the ADC input dynamic range is properly utilized. If this
forced offset were not imposed, the ADC output codes would
be reduced to half of what the ADC is capable of producing,
resulting in a significant reduction in ENOB. The choice of the
direction of this shift is determined by the polarity of the ex-
pected signal.
Another scenario where it may be necessary to shift the
LMH6555 output offset voltage is in applications where it is
necessary to improve the specified Output Offset Voltage
(differential mode), “V
ADC081000/ ADC081500 (and their dual counterparts), have
internal registers to correct for the driver’s (LMH6555) V
If the LMH6555 V
lowed into this register, then shifting the output is required for
maximum ADC performance.
It is possible to affect output offset voltage by manipulating
the value of one input resistance relative to the other (e.g.
R
the gain. Assuming that the source is applied to the V
through R
the overall gain and output offset voltage. Figure 7(B) shows
the same effects but this time for when the undriven side
impedance, R
S1
relative to R
FIGURE 6. Input Voltage for Figure 5 Schematic
S1
, Figure 7(A) shows the effect of varying R
S2
, is varied.
S2
OOS
or vice versa). However, this will also alter
rating exceeds the maximum value al-
OOS
S1
S1
”. Some ADC’s, including the
and R
and R
S2
S2
, unless otherwise spec-
, as shown in Figure 3
20127765
www.national.com
IN
+
S1
OOS
side
on
.